1. Field of the Invention
This invention relates to an improvement in isotope separation processes that employ a selective photon-induced energy level transition of an isotopic molecule containing the isotope to be separated and a chemical reaction with a chemically reactive agent to provide a chemical compound containing atoms of the desired isotope. The invention discloses certain molecular attributes which is possessed by a chemically reactive agent used in the aforementioned process, make it more effective and the isotope separation more economical.
2. Description of the Prior Art
The application of lasers for isotope separation has been the subject of many studies and development programs in the last two decades. In particular, the separation of U-235 and U-238, needed for nuclear reactor fuel, has received considerable attention and has led to three distinct approaches. These approaches use lasers to cause isotope-specific ionization (AVLIS), dissociation (MOLIS), or activation of a chemical reaction (CRISLA).
In the AVLIS approach, which is an abbreviation for Atomic Vapor Laser Isotope Separation, isotopic metal is vaporized (usually by means of electron guns) and the vapor is irradiated by two ultraviolet or three visible superimposed laser beams at two or three different wavelengths. In one AVLIS scheme applied to the separation of Uranium and developed by the U.S. DOE at the Lawrence Livermore Laboratory, a copper vapor laser is used as the primary source of (green) laser photons. Dyes are used to convert these protons to certain visible frequencies required for efficient three-step selective excitation and ionization of U-235 atoms. The selectively ionized U-235 ions are next removed from the U-238/U-235 vapor by electromagnetic fields. This process is discussed in "Laser Spectroscopy and its Applications," edited by L. J. Radziemski, R. W. Solarz, and J. A. Paisner; Marcel Dekker, Inc. N.Y. (1987), at pages 235, et seq., hereinafter "Laser Spectroscopy."
In the MOLIS technique, which is an acronym for Molecular Laser Isotope Separation, gaseous isotopic molecules are employed instead of metal vapors. For example, in a Uranium enrichment technique developed by the U.S. DOE Los Alamos Laboratory, gaseous Uranium Hexafluoride (UF.sub.6) is used and irradiated with two or three successive 16-micron laser photons causing isotope-selective excitation of .sup.235 UF.sub.6 to the 2.nu..sub.3 or 3.nu..sub.3 vibrational level as stated in Laser Spectroscopy, at pages 459, et seq. The 2.nu..sub.3 or 3.nu..sub.3 -excited .sup.235 UF.sub.6.sup.* is next irradiated with an ultraviolet (UV) laser beam causing it to dissociate to UF.sub.5 +F. Instead of using a UV laser beam, the isotope-selectively excited .sup.235 UF.sub.6.sup.* can be dissociated by a second high-energy 16-micron infrared (IR) laser pulse which causes multi-photon absorption and dissociation. Thus, some MOLIS schemes use two or three isotope-selective 16-micron IR laser pulses followed by a UV laser pulse, while others use two or three isotope-selective 16-micron IR laser pulses followed by a high-energy second (red-shifted) 16-micron pulse that causes dissociation by multi-photon absorption.
In CRISLA, which stands for Chemical Reaction by Isotope Selective Laser Activation, one laser beam is used which irradiates a gaseous mixture of the isotopic molecule to be separated (e.g., UF.sub.6) and a coreactant RX. In the case of UF.sub.6, for example, as described in U.S. Pat. No. 4,082,633, a mixture of UF.sub.6 and a suitable coreactant RX is isotope-selectively irradiated by 5.3 micron CO laser photons in an intracavity reaction cell. In this process, .sup.235 UF.sub.6 is preferentially excited over .sup.238 UF.sub.6 to the 3.nu..sub.3 vibrational excitation level. The excited .sup.235 UF.sub.6.sup.* molecules react much more rapidly with the coreactant RX than unexcited UF.sub.6, resulting in a Uranium-bearing reaction product that is enriched in .sup.235 U.
Both MOLIS and CRISLA depend upon vibrational molecular isotope shifts of hot-banded absorption contours. The overlap of the isotopic bands is generally smaller, the colder the irradiated gas is. This means that higher separation factors are obtained at lower temperature. However, UF.sub.6 has a very low vapor pressure at the desired lower temperatures, causing throughputs to be very low. To overcome this problem in the MOLIS process, a mixture of UF.sub.6 and a carrier gas such as Helium, Argon, Nitrogen, Hydrogen, or Methane, is usually used and supercooled in an expanding supersonic jet. the jet is then intercepted by a 16-micron laser beam at a point where the UF.sub.6 is still gaseous but far below its normal condensation temperature. Although supersonic jet-cooling could also be used in CRISLA, because of the higher isotope shift at =5.3 micron used in CRISLA, arrangements that require only static or limited adiabatic-expansion cooling are usually adequate. In Uranium enrichment by the CRISLA technique, the preferred wavelength is 5.3 micron at which the isotope-shift between the .sup.235 UF.sub.6 and .sup.238 UF.sub.6 absorption bands is three times larger than at 16 micron. On the otherhand, the UF.sub.6 absorption cross-section at 5.3 micron is 10,000 times less than at 16 micron.
The lasers used in the AVLIS and MOLIS Uranium enrichment schemes are pulsed so that different frequencies are absorbed at different times with time frames and intervals that range from nanoseconds to milliseconds. In Uranium enrichment with CRISLA, on the otherhand, only one (or two) continuous-wave (CW) laser beam(s) is (are) employed and no time-gating is required. The result is that the laser systems used in CRISLA are much simpler and less costly than those used in AVLIS and MOLIS. On the otherhand, CRISLA requires the use of a suitable chemical reaction which adds cost and complexity to the subsequent physical separation of product and unreacted UF.sub.6. The proper choice of an effective coreactant is CRISLA is, therefore, desired so that a more efficient process is obtained.
In CRISLA, chemical energy is used for most of the separation work, whereas in AVLIS and MOLIS, all the energy provided for separation is photonic. The attractiveness of CRISLA over AVLIS and MOLIS is in part due to the fact that chemical energy is generally less expensive than laser photon energy. The techniques of photon-induced ionization and dissociation used in AVLIS and MOLIS rely on straight-forward extrapolations of earlier developed scientific knowledge. For this reason, investigations of these laser isotope enrichment processes were completed earlier than CRISLA.
The desired coreactant in the CRISLA process is a coreactant RX which in its complexed state with a Uranium-bearing laser-excited molecule UY.sup.*, that is in the molecular complex UY.sup.* :RX, shows a high reaction sensitivity to the vibrational excitation of the bond U--Y. In certain particular cases, the photon energy E.sub.L =hV.sub.L pumped into UY.sup.* is insufficient to overcome the reaction barrier energy E.sub.a, that is E.sub.L &lt;E.sub.a. However, if in this case E.sub.L &lt;E.sub.a &lt;2E.sub.L, it is essential that the coreactant RX also absorb a laser photon E.sub.L =h.nu..sub.L so that the total pumped energy in the complex UY.sup.* :RX.sup.* is doubled to 2E.sub.L and reaction is promoted. In addition to reaction sensitivity, it is important that the isotope-carrying product formed in a CRISLA reaction does not engage in subsequent chemical scrambling. Therefore, it has long been desired to define certain essential molecular properties and selection criteria for RX that will ensure efficient isotope-selective laser-induced reactions of the complex UF.sub.6.sup.* :RX or UF.sub.6.sup.* :RX.sup.* and the formation of UF.sub.5 X products that undergo little or no subsequent chemical scrambling. Application of these selection criteria to all reactable RX molecules, greatly restricts the number of RX molecules that are useful in particular applications of the CRISLA process. Thus, by employing coreactants from this limited predefined group, considerable improvements in the CRISLA process result. The selection criteria can be equally applied to the CRISLA enrichment of isotopes other than Uranium.